1. Field of the Invention
This invention relates to a process for the preparation of N-acylaminomethylphosphonic acids. More particularly, the invention relates to the production of N-acylaminomethylphosphonic acids from carboxylic acid amides, a suitable formaldehyde source, phosphorous acid, and a carboxylic acid anhydride in the absence of halogenated reactants, whereby possible contamination of the reaction mixture by carcinogenic halomethylethers is avoided.
The N-acylaminomethylphosphonic acids are intermediates to aminomethylphosphonic acid, and both classes of compound are intermediates to N-phosphonomethylglycine, as is disclosed in U.S. Pat. Nos. 5,041,627, 4,470,191 and 4,851,159 and in European Patent Application 186,648.
N-phosphonomethylglycine, commonly known as glyphosate, is a broad spectrum phytotoxicant or herbicide useful in controlling the growth of a wide variety of plants.
2. Related Prior Art
Numerous methods are known for the preparation of N-acylaminomethylphosphonic acids. Various such methods are reviewed in the above-referenced U.S. Pat. No. 5,041,627 by Baysdon and Fields, which patent is hereby incorporated herein by reference. Patentees Baysdon and Fields teach that prior processes that involve reaction of N-methylolcarboxamide intermediates suffer from low yields or problems associated with handling unstable methylol derivatives. They accordingly propose and claim a process for the preparation of N-acylaminomethylphosphonic acids which comprises bringing together under substantially anhydrous reaction conditions a suitable carboxamide and paraformaldehyde, thereafter adding phosphorous trihalide to form a reaction mixture, heating the reaction mixture and adding water to form the desired N-acylaminomethylphosphonic acid.
Whatever the merits of the disclosed process, the use of phosphorous trihalide, e.g., phosphorous trichloride, entails the serious disadvantage that there is the likelihood of producing reaction mixtures contaminated with carcinogenic halomethylethers.
In this connection, Kleiner in WO/03448, published May 3, 1992, states that processes involving treatment of N-hydroxymethylamides with phosphorous trichloride followed by hydrolysis with hydrochloric acid are highly disadvantageous in that they produce the carcinogenic bischloromethylether as a byproduct. Kleiner proposes instead to prepare aminomethylphosphonic acid by reaction of a suitable N-hydroxymethylcarboxamide with phosphorous acid in the presence of at least equimolar quantities of acetic anhydride to form a reaction product (undescribed) followed by heating the reaction product with large quantities of water at elevated temperatures for extended periods of time.
The Kleiner process itself suffers in that it requires the use of preformed N-hydroxymethylcarboxamides as starting materials, and large excesses of water at elevated temperatures for long periods of time in order to obtain the desired aminomethylphosphonic acid.
Thus, a need exists for a N-acylaminomethylphosphonic acid process that avoids the need to use a preformed N-hydroxymethylcarboxamide starting material and eliminates the possibility of the formation of undesirable halomethylether byproducts. A need also exists for an improved process for hydrolyzing N-acylaminomethylphosphonic acid to aminomethylphosphonic acid.
Another aspect of the invention comprises an improved hydrolytic process wherein the N-acylaminomethylphosphonic acids are effectively and efficiently hydrolyzed to aminomethylphosphonic acid by treatment with dilute(3-10% by weight) mineral acid, e.g., hydrochloric or sulfuric, under relatively mild conditions of temperature and time, e.g., 100.degree. to 130.degree. C. in less than 4 hours, in marked contrast to prior art procedures involving concentrated acid (e.g., concentrated (37%) hydrochloric acid) for 16 hours at reflux or water alone at 200.degree. C. for 20 hours.